1. Field of the Invention
The present invention relates to a fixing device for fixing an image onto a printing medium by applying heat and pressure thereto, and an image forming apparatus, such as a copier, a printer, a facsimile, a multi-functional machine, etc., having the fixing device.
2. Discussion of the Background Art
An image forming apparatus usually employs a fixing device that fixes a toner image on a printing medium by applying heat and pressure thereto. The fixing device generally includes a fixing roller heated by a heat source and a pressing roller pressing against the fixing roller. Specifically, the toner image is fixed onto the printing medium when the printing medium with the toner image is conveyed through a fixing nip created between the fixing roller and the pressing roller.
However, due to melting and adherence of the toner to the fixing roller during such a fixing process, especially when a print rate is high, it can happen that the printing medium winds around the fixing roller and offset of a toner image appears, for example.
Then, many fixing devices have been proposed to improve a separation performance of separating the printing medium from the fixing roller. For example, as shown in FIG. 16, a conventional fixing device includes a fixing roller 100, a pressing roller 200, a separation roller 300, and an endless fixing belt 400 wound around the separation and fixing rollers. The pressing roller 200 presses against the fixing belt 400 while opposing the fixing roller 100 and creating a fixing nip N there. Further, the fixing roller 100 receives a driving force from a driving source, not shown, and rotates in an arrow A showing direction. Thus, as the fixing roller 100 rotates, the fixing belt 400 travels in an arrow B showing direction, and the separation roller 300 and the pressing roller 200 are driven and rotated in arrow C and D showing directions, respectively.
When a sheet P with a transferred toner image T enters the fixing nip N, the toner image T is heated, pressed, and fixed thereonto. Then, the sheet P is conveyed by the fixing belt 400 and is separated by the separation roller 300. Thus, the fixing device cools down the toner on the sheet P during conveyance on the fixing belt to facilitate separation of the sheet P therefrom.
The fixing belt sometimes expands when heated. Thus, not to slack even causing the thermal expansion, the fixing belt 400 needs a prescribed tension. In such a situation, a tension roller can be employed beside the fixing roller 100 and the separation roller 300 to create the prescribed tension on the fixing belt 400. However, number of parts increases and the apparatus becomes bulky. To avoid such a problem, the separation roller 300 can be made to function as the tension roller.
Specifically, the separation roller 300 may be moved to either approach or withdraw from the fixing roller 100 in the arrow E showing direction by biasing the separation roller 300 toward the fixing roller 100 using a spring, not shown, and applying tension to the fixing belt 400.
However, since the fixing roller 100 rotates while the separation roller 300 is driven and rotated, the fixing belt 400 tends to slacken at a position F in which the sheet P is conveyed from the fixing nip N to the separation roller 300. When the fixing belt 400 slackens, the sheet after the fixing process cannot tightly contact the same, and accordingly, an offset image or unevenness of gloss appears.
Another conventional fixing device provides a separation roller 300 with a driving force to rotate the fixing roller 100. Thus, the fixing belt 400 is stretched as it travels suppressing slack in the path F, so that image offset or gloss unevenness can be suppressed.
As mentioned above, to effectively obtain a fine image without slack in the fixing belt, the separation roller preferably serves both as a tension roller and a driving roller.
Further, a fixing device having a separation roller 300 sometimes serves both as the tension and driving rollers as illustrated in FIG. 17. Specifically, a driving gear 600 is integrally attached to the separation roller 300, and is meshed with a power transmission gear. Thus, a driving force from a motor, not shown, is transmitted to the driving gear 600 via the power transmission gear 500 to rotate the separation roller 300. Further, the separation roller 300 is arranged to either approach or withdraw from the fixing roller 100 in the arrow E showing direction. The separation roller 300 is biased by a spring, not shown, and withdraw from the fixing roller 300 so that the fixing belt 400 has a prescribed tension.
Thus, as shown in FIG. 18, when the separation roller 300 moves along a straight line from the position shown by a solid line to that shown by a broken line, a relative distance between the rotational center O20 of the separation roller 300 and that of O10 of the power transmission gear 500 changes from M1 to M2. Consequently, a condition of meshing of the driving gear 600 with the power transmission gear 500 becomes uneven, rotational force is not stably transmitted to the separation roller 300, and unevenness of rotation occurs. As a result, image quality deteriorates and the life of the gear is shortened.